chondrichthyans

Chondrichthyans . Class of fish with a cartilaginous skeleton, more or less calcified, but never bony, with skin covered with placoid scales, called dermal denticles, without a swim bladder , with a spiral valve in the intestine and, generally, without an operculum, five or more gill openings on the sides of the head , heterocercal caudal fin and lower mouth.

Summary

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• 1 appearance
• 2 Scientific classification
• 3 Sexual dimorphism
• 4 Playback
• 5 Division of chondrichthyans
  • 1 The squaliforms
  • 2 The batoids or rayiforms
• 6 sources

Appearance

During the Devonian Period the first fish with a cartilaginous skeleton appeared. Sharks, rays and chimeras derived from them. As its name implies, its skeleton is made of cartilage, a flexible and resistant substance but less hard than bone . The body of this type of fish has two groups of paired fins: scapular or anterior, and pelvic or posterior. The mouth has jaws and teeth that usually open on the ventral side of the head.

They have a pair of nasal openings located on both sides of the face and eyes , behind which there is a hole called a spiracle, which connects the pharyngeal cavity with the outside and serves as a circulation route for the water that passes through the branchial apparatus. The skin is almost always covered by dermal denticles or placoid scales, which, being very close together, produce a sensation of great roughness to the touch.

In the vicinity of the mouth, these same scales give rise to the teeth . A very important characteristic within this group of fish is the diet, being the majority carnivorous (there are species that eat plankton, and there are even some vegetarians), and the lack of a swim bladder; that is, the organ that allows them to float.

scientific classification

Class that includes the subclasses elasmobranchs and holocephalians; includes cartilaginous fish with well-developed jaws (sharks, etc.); they are covered by a rough skin, and have rigid fins that cannot be folded. Its powerful jaws are equipped with an endless supply of crushing teeth. Opposes Osteichthyos .

sexual dimorphism

In some cases there is sexual dimorphism, which is presented in characteristics such as the presence of sexual organs in the male, smaller size compared to the female and different dentition. Fertilization is internal and the number of eggs is quite small. The eggs are large and have a shell that varies in shape and size depending on the species. Certain types of chondrichthyans are ovoviviparous, which means that they do not lay eggs externally, but the females incubate them internally and give birth to an adult-like offspring.

Reproduction

Fertilization is internal. The pterygopods on the fins of the males are copulatory organs. The male introduces one of the pterygopods into the female’s cloaca, where the sperm is sucked into the oviduct. A strong tendency is observed in the whole group to ovoviviparism. In certain cases, oviparism or even viviparism occurs.

The phenomenon of “oophagia” (cannibalism inside the uterus , where an embryo eats remains of eggs) is also presented.

chondrichthyan division

Cartilaginous fishes can be divided into two groups: plagiostomes and holocephalians . The former are characterized by having five or more gill openings; the latter, meanwhile, only have one external gill opening on each side. Within the plagiostomes we find the squaliforms and the batoids or rayiforms.

the squaliforms

They have five to seven gills located on the sides of their heads, their bodies are fusiform, their teeth are sharp or sharp, and their fins well developed. The most typical sharks are sharks, which can become large and extremely voracious, even with humans. However, there are also other quite minor species, which are not dangerous because they feed on small prey, such as molluscs and small fish. Other categories appear within the squaliformes:

• Hexanchids: presence of a single fin on the back.
• Scylids: presence of two fins on the back, located very close to the anal region (species of this type: catshark).
• Isurids: large size and powerful dentition. Wide anterior and caudal fins (species of this type: mako shark, silky shark).
• Allopids: extraordinary development of the caudal fin, as long as the rest of the fish (species of this type: foxfish ).
• Cetorrhinids: fusiform body, pair of highly developed dorsal and caudal fins (species of this type: peregrine or whale shark).
• Galeidae: presence of a membrane on the inner side of the lower edge of each eye (species of this type: blue shark).
• Sphinxes: sides of the head with extensions to the sides that come to form a transverse piece (species of this type: hammerhead fish).
• Skeletons: lack of anal fin (species of this type: spiny dogfish).
• Scymnorhinids: lack of anal fin, without spines on the fins (species of this type: rockfish, Greenland shark ).

The batoids or rayiforms

Although most rays live in the deep, some also move in shallow water. They have a body whose nasal openings and five pairs of gills are located on the ventral side. The eyes and spiracles are on the dorsal surface of the head. The pectoral fins are noticeably widened and merge with the trunk and head, giving the body a flared appearance. The unpaired fins are located on the tail and are small in size. They swim by means of undulating movements of the wide pectoral fins, although they are normally perched and camouflaged on the seabed. Their sedentary-type feeding is achieved by unexpectedly attacking their prey. In the group of batoids we find the following:

• Rinidae: one could say that they are transitional forms between sharks and batoids (a species of this type: angelfish).
• Pristids: extended face in a long, flattened appendage (species of this type: sawfish).
• Rhinobatids: they are commonly called guitar, due to their widened shape that resembles this resonance box (species of this type: Rhinobatus rhinobatus).
• Torpedinids: trunk and pair of front fins that take the shape of a rounded disk. They have the ability to accumulate a strong electrical potential (kind of this type: tremielga, tremielga).
• Rayids: head, trunk and fins of the first pair unite to form a disc with thin edges and almost rhomboid contour (species of this type: rough ray, painted ray).
• Trigonids: body similar to rays, but with a tail similar to a long and flexible whip (species of this type: mantellina).
• Miliobatidae: large pectoral fins similar to the wings of a bird (species of this type: sea eagle ).
• Mobrilids: pair of extensions similar to horns on the front of the head (species of this type: manta fish).

The second group of chondrichthyan fish corresponds, as we said, to the holocephalians, within which the so-called chimerids appear. They resemble bony fish in that they have an external gill opening on each side, but their appearance is quite particular: a voluminous head from which the body elongates and tapers until the caudal region becomes a kind of flexible whip. Chimerids are oviparous. The egg is large and provided with a capsule with a horny texture made up of three portions that correspond to the cephalic, trunk and caudal regions of the embryo.